1. Field of the Invention
This invention relates to a tread rubber composition for pneumatic tires, and more particularly to a tread rubber composition suitable for use in pneumatic tires, such as passenger car radial tires, passenger car bias tires, motorcycle tires and so on, which run at a high speed of, for example, more than 200 km/hr, while maintaining a high friction force to the road surface or a high gripping force.
2. Related Art Statement
Lately, studies on high-speed running tires have been carried out in parallel with studies on speeding up of automobiles due to the improvement of roads, particularly express ways. In this connection, studies on tread rubber in the tire have also been made. It is well-known that tread rubbers having a large hysteresis loss during the deformation are used for increasing a friction force between the tread and road surface of a gripping force in the pneumatic tire. In order to increase the hysteresis loss of the tread rubber, styrene-butadiene copolymer rubber having a large bound styrene content is used or a large amount of carbon black is compounded into the tread rubber. However, it is also well-known that as the hysteresis loss of the tread rubber becomes large, the heat generation of the tread rubber becomes high during the running of the tire and hence the breakage of the tread rubber is apt to be caused due to blow-out or the like. Therefore, the feature of obtaining a large gripping force between the tire and road surface and feature of obtaining a high durable performance in high-speed running are conflicting requirements from a viewpoint of plans for the tread rubber because the improvement of one of the features results in the degradation of the other feature. For this reason, it is very difficult to obtain tread rubbers simultaneously satisfying the large gripping force and high durable performance in high-speed running.